As is known, in such feeders, the weft yarn inserted in the loom shed is stopped, after having reached the preset length, by a controlled device usually comprising a movable rod which engages the edge of the drum by penetrating with slack into a hole formed on said drum, the weft yarn stopping against said rod and being braked thereby.
The present invention proposes to solve the problems arising just after the weft yarn has been stopped and, in particular, to provide for an efficient blocking of said yarn just after it has been cut by the appropriate cutting devices of the loom.
It should be remembered that, for the loom cutting devices to correctly perform their function, it is necessary for the weft yarn length extending between the measuring weft feeder and the fabric being woven to be sufficiently tensioned at the moment in which it is being cut; consequently, since just after cutting there is a sudden decrease in weft yarn tension, this causes oscillations on the yarn starting from its cut end and travelling backwards to the point in which the yarn is blocked by the rod of the stopping device on the weft feeder drum. These oscillations may even continue up to involving the yarn turns wound on said drum.
These oscillations, turning on the yarn into strong transversal and longitudinal waves, may cause various inconveniences, such as:
a) Wedging-in of the weft yarn beneath the rod of the stopping device, when this latter is already acting into the drum hole. PA1 b) Ruffling of the yarn turns wound on the drum, with possible overlapping thereof, whereby, during the next weft yarn insertion, the turns will no longer unwind regularly one at a time from the drum, but may be drawn by two or more turns at a time getting entangled at the outlet of the drum.
It is evident that, both in the first case and in the second case, measuring errors might occur, which may in turn cause stopping of the loom, thereby reducing the efficiency of the machine, or else give rise to faults, which are not detected by the loom control devices and will thus deteriorate the quality of the fabric being produced.
To try and overcome the problem mentioned in a), the applicant has already developed in the last years different improvements in the electromagnetic weft yarn stopping units adopted on measuring weft feeders for fluid jet looms: the most interesting solutions are those described and claimed in IT-U-209674, and in the recent EP-363938.
An attempt to solve the problem mentioned in b) had instead been made, always by the applicant, by adopting an additional device mounted in correspondence of the yarn guide eyelet at the outlet of the measuring weft feeder: this solution is described in EP-239055. Nevertheless, said device was not able to fully solve the problem in that, since the weft yarn was not blocked but merely deviated, it was still likely to loosen around the drum.